1. Field of the Invention
The present invention relates to a method and an apparatus for removing harmful components in an exhaust gas derived during manufacturing an electronic circuit element such as a semiconductor device or a liquid crystal device, particularly during a cleaning or etching process. Further, the present invention relates to an apparatus that is applicable to remove harmful components in a gas generated during the process of smelting aluminum.
2. Description of the Prior Art
In a semiconductor manufacturing apparatus such as a CVD apparatus, a gas for deposition (such as SiH4, Si2H6, SiH2Cl2, TEOS, PH3, B2H6, NH3, N2O, or the like) is used in forming various thin films, and a cleaning gas (such as NF3, C2F6, CF4, SF6, or the like) is usually used for cleaning the inside of the semiconductor manufacturing apparatus after completing the deposition process.
These gases inherently have various dangerous factors such as flammability, explosiveness, corrosiveness, poisonousness, and the like. Therefore, it is required to remove(detoxify) harmful components in these gases using a harm-removing apparatus equipped, for example, with a means for oxidizing and heating the gases before they are released into atmospheric air.
In the semiconductor manufacturing apparatus such as a CVD apparatus, complex decomposition reactions occur in the gases that are being used, so that new decomposition products (such as F2, HF, and SiOx) are generated as a result and these decomposition products are discharged together with the undecomposed deposit gas and the cleaning gas.
In the semiconductor manufacturing process, a semiconductor manufacturing apparatus such as a CVD apparatus generally operates as follows: deposition using a deposition gas such as SiH4 (toxic to a human body and explosive)xe2x86x92purging of residual SiH4 gas from the CVD chamber using nitrogenxe2x86x92cleaning the CVD chamber using a cleaning gas such as C2F6 (harmless to a human body though exhibiting a greenhouse effect)xe2x86x92purging of the cleaning gas from the CVD chamber using nitrogenxe2x86x92repeating this cycle.
Here, one group of said gases to be used for cleaning of a CVD chamber is what is known as PFC gases. PFC is an abbreviation for xe2x80x9cperfluorocarbonxe2x80x9d or xe2x80x9cperfluoride compoundxe2x80x9d. Representative examples of PFC are CF4, CHF3, and said C2F6. If the term xe2x80x9ccompoundxe2x80x9d is used instead of xe2x80x9ccarbonxe2x80x9d, PFC will further include fluorine compounds that do not contain carbon, such as NF3, SF6, and SF4.
The purpose of the present invention is to establish a technique for removal of the former PFC gases, which technique has not yet reached an applicable level for use in a removal device or a removal method. Naturally, however, the technique of the present invention is also applicable for removal of all the PFC gases including the latter PFC gases.
The PFC gases as represented by CF4 and C2F6 are non-flammable and toxicity of the gases themselves on human beings is unknown. At least, acute and subacute toxicities are not known. However, since the compounds themselves are stable, they remain unchanged and stay for a long period of time if they are released to an ambient atmosphere. It is reported that the life span until consumption in the atmosphere is 50,000 years in the case of CF4, and 10,000 years in the case of C2F6.
Further, the global warming factor (relative to CO2) is 4,400 in the case of CF4 and 6,200 in the case of C2F6 (after 20 years have passed), which involves a problem that cannot be left aside in considering the earth environment. Therefore, it is eagerly desired to establish a means for removing the PFC gases as represented by CF4 and C2F6.
However, the former PFC gases, i.e. the compounds as represented by CF4, CHF3, and C2F6, have a stable Cxe2x80x94F bond (having a bonding energy as large as 130 kcal/mol) and are not easily decomposed, so that it is extremely difficult to remove them by simple thermal oxidation decomposition.
For example, in the case of C2F6, the decomposition proceeds by a cut in the Cxe2x80x94C bond, so that C2F6, can be removed by limiting the processing flow rate to be less than 250 liters/min at a processing temperature of 1000xc2x0 C. However, in the case of CF4, it is necessary to cut the Cxe2x80x94F bond that has the largest bonding energy, so that a temperature of 1400xc2x0 C. is required even with the above-mentioned flow rate. In addition, even by the above method, it is difficult to remove more than 80% of the total gas.
Further, if an electric heater is to be used, attainment of a high temperature atmosphere of more than 1400xc2x0 C. is an upper limit also from the view point of materials for the heater, so that a long-term usage is almost impossible. Moreover, maintaining the temperature of the entire apparatus is also difficult and, in combination with a thermal insulating material, the total volume of the apparatus will be large and it will not be a compact apparatus. What is more important is that the thermal energy cost will be excessively high.
Here, in this field, the following new method has been proposed. International Publication Number WO94/05399#Method of Decomposing Gaseous Halocarbon# reports that coexistence of O2 makes it possible to decompose and remove, for example, CF4 at a temperature of 600 to 700xc2x0 C. However, a detailed follow-up experiment of the contents of the publication turned out to be a complete failure in removal under this condition.
Also, an attempt is made to positively introduce H2 gas to pyrolyze PFC. However, it requires a high processing temperature and, besides, it may not be suitable for use from the view point of safety, since the H2 gas is flammable and explosive.
The present invention is intended to develop a harm-removing apparatus capable of decomposing and removing PFC components at a high removal ratio at a temperature as low as possible (with thermal energy consumed at an amount as small as possible). In other words, the present invention provides a method and an apparatus for removing PFC components at a low temperature, removing the derived fluorine components by separately washing or fixing them, and releasing the other components into the atmospheric air basically as CO2 and H2O. The essence of a PFC removing method according to the present invention is bellow:
The method of the present invention mixes at least one of a hydrocarbon gas and NH3 gas with an exhaust gas containing a perfluorocarbon or a perfluorocompound and thermally decomposes the mixed gas in a non-oxidizing atmosphere at a temperature (600-1300xc2x0 C.) lower than the temperature at which the conventional apparatus and method performs thermal decomposition.
The term xe2x80x9cnon-oxidizing atmospherexe2x80x9d herein used will be described later in detail, but means, in a word, an atmosphere free of oxygen in decomposing of the exhaust gas.
When the exhaust gas is washed with water prior to the thermal decomposition, it is possible not only to remove soluble components, dust and the like in the exhaust gas as discharged from manufacturing equipment prior to the thermal decomposition but also to allow the exhaust gas to be incorporated with water content so as to cause the perfluorocarbon or perfluorocompound contained in the exhaust gas to be thermally hydrolyzed.
Where the hydrocarbon gas is mixed excessively, the gas thermally decomposed comes to contain unreacted hydrocarbon gas and soot resulting from the decomposition reaction. Such unreacted hydrocarbon gas and the soot are removed by burning in the next step. Therefore, the term xe2x80x9cflammable componentsxe2x80x9d used herein is meant to include an excess of the hydrocarbon gas and soot.
For instance, in the case of thermal decomposition of CF4 and C2F6 using C3H8 in a non-oxidizing atmosphere, the decomposition proceeds as follows:
C3H8xe2x86x923C+8H (radicals) . . . decomposition at the gas decomposer room;
CF4+4H (radicals)xe2x86x92C+4HF . . . decomposition at the gas decomposer room;
C2F6+6H (radicals)xe2x86x922C+6HF . . . decomposition at the gas decomposer room;
C (soot)+O2xe2x86x92CO2 . . . burning at the burner room; and
6HF . . . removed at the second scrubber or the adsorber tower.
Further, any fluorine compound resulting from the thermal decomposition is removed by washing with water or chemical adsorption. Such washing or chemical adsorption process may be performed prior to the burning process, or vice versa.
Major unit operations in the invention are the following three steps.
(a) Thermal decomposition of PFC
(b) Exhaustion by washing or removal by fixing of the generated fluorine compounds
(c) Removal by burning of other flammable components
An important feature of the present invention is the thermal decomposition of PFC of the above step (a) and aims at establishing a technique for processing a gas at a PFC removal ratio of 90% or more in a temperature region (600xcx9c1200xc2x0 C.) considerably lower than the atmosphere temperature required for an ordinary simple thermal decomposition. The PFC removal ratio is calculated as (Axe2x88x92B)÷Axc3x97100, where A and B represent the PFC concentration in the gas introduced into the removing apparatus and the PFC concentration in the released gas, respectively. When the thermal decomposing temperature is at under 600xc2x0 C. a PFC removal ratio goes down to 20xcx9c30%,which is not suitable for use. On the other hand at more than 1200xc2x0 C., consumption of heat energy needs so much that the heater is impossible to be continuously used for a long period of time. It is like to get a higher thermal decomposing temperature with perfluorocarbon and a lower temperature with perfluoride compound.
The thermal decomposition of PFC that forms an essence of the present invention may employ an electric heater as a heat source or a burner using a liquid fuel such as LPG (Liquefied Petroleum Gas) or LNG (Liquefied Natural Gas) or a gaseous fuel such as CH4, H2, or CO as a heat source. The heat source for heating the gas to be processed is positioned at the gas decomposer room. The electric heater to be used as the heat source may be disposed outside of the gas decomposer room for heating the gas decomposer room from outside or may be disposed inside of the gas decomposer room 12, 22 for directly heating and decomposing the introduced exhaust gas containing PFC. If the liquid fuel or the gaseous fuel is to be used as the heat source, the gas decomposer room is heated from outside only.
Into a space within the gas decomposer room, a mixed gas containing PFC as a major component and typically N2 as a carrier gas is introduced, and further at least one of saturated or unsaturated C1-C8 hydrocarbon gas and NH3 gas in a gaseous state is simultaneously supplied. In this case, if an O2 gas or an O3 gas is used in combination in the semiconductor manufacturing apparatus, the remaining O2, O3 gas flows as an exhaust gas into the gas decomposer room. However, besides the above remaining O2 or O3 gas, neither an O2, O3 gas nor an external air is intentionally introduced into the gas decomposer room. Therefore, one of the essential conditions in the present invention is that at least the inside of the gas decomposer room is not an oxidizing atmosphere. This state is hereafter referred to as xe2x80x9cabsence of separated O2 or O3xe2x80x9d. In other wards, separated O2 or O3 refers to what are left in an exhaust gas without consuming depend on the reaction in the semiconductor manufacturing apparatus.
Under this condition, PFC is decomposed by appropriately adjusting the PFC concentration in the gas to be processed, the flow rate of the gas to be processed, and the spatial temperature in the gas decomposer room. In this case, the hydrocarbon introduced as an agent for decomposing PFC is thermally decomposed in a non-oxidizing atmosphere. For example, if propane is used, various decomposition products such as methane, ethane, ethylene, and propylene are generated, and it has been found out that the fluorine components are isolated as F2 or HF due to the interaction between one of the above decomposition products and PFC.
Also, it has been confirmed that the PFC can be almost completely removed even if the temperature of the atmosphere is lower by several hundred degrees than the temperature in processing PFC alone or in an oxidizing atmosphere system.
Without introducing the hydrocarbon agent, it is extremely difficult to attain 80% or more of the PFC removal ratio in processing PFC alone or in an oxidizing atmosphere system even if a high temperature region, which is near the upper limit for the material to be used, is employed. This shows that the mechanism of decomposition of the compounds without introducing the hydrocarbon agent in the gas decomposer room is completely different from that of the present invention.
The processed gas discharged from the gas decomposer room according to the present invention contains F2 or HF gas, a gas generated by decomposition of the hydrocarbon as the decomposing agent, and possibly carbon soot depending on the processing condition.
In the present invention, a fluorine-based exhaust gas and a flammable gas are respectively subjected to harm-removal processes by introducing the two components into separate processing towers in series.
Namely, the former (fluorine-based exhaust gas) is separated and discharged out of the system either by allowing it to be absorbed and dissolved in water by passing it through the second water scrubber or by allowing it to be chemically adsorbed onto a solid adsorbing agent of CaO or CaCO3. The latter (flammable gas) is burnt in the presence of external air and the processed final gas is released into atmospheric air.
These, and other objects and advantages of the present invention will become more evident to those skilled in the art from a consideration of the following detailed description of the preferred embodiment, particularly when read in conjunction with the appended drawings, a brief description of which now follows.